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remember a formula

Discussion in 'Electronic Basics' started by js5895, Apr 6, 2005.

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  1. js5895

    js5895 Guest

    Hi,

    I'm studying electrical, what's the best way to remember the P.I.R.E.
    wheel.

    Thanks.
     
  2. Lord Garth

    Lord Garth Guest

    Learn just one formula such as V=IR and rearrange terms mathematically
    as needed. No escaping the math if you want this field.
     
  3. Pedantic mode on. :)
    You have to learn _two_ equations, actually.

    V=I*R (Volt=Amp*Ohm)
    P=U*I (Watt=Volt*Amp)

    Then learn how to re-arrange these equations as needed for the problem
    at hand, and use a calculator to get the result.

    The art of re-arranging equations is called algebra, and you need some
    basic knowledge and experience in this.


    An alternative is to use a visual diagram like the ones I have put on a
    web site

    http://humanist.250free.com/

    Click on the two .jpg files at the bottom of the list, save them to
    hard disk. Can be distributed freely.
     
  4. John Smith

    John Smith Guest

    If you going to learn P=VI rather learn P=VIcos(Phi) where Phi is the phase
    between V and I.
     
  5. Tom Biasi

    Tom Biasi Guest

    Hi,
    You are asking for a method to' remember' something and you are being given
    answers on how to' learn' something.
    I would suggest that you take the advice that says 'learn' algebra. If you
    learn the relationship that is generically referred to as "Ohm's Law" and
    learn the algebra to solve for all variables you will be in far better shape
    when the formulae get more complicated.
    Regards,
    Tom
     
  6. BobG

    BobG Guest

    Can someone sum up the top couple of rules of algebra for him? How
    about something like: 'An equation has an expression on each side of
    the equal sign. To solve the equation for any of the variables, you
    need to get that variable over to the left side of the equal sign. To
    eliminate a variable on one side, multiply both sides of the equation
    by the inverse of that variable. This doesnt change the equality,
    because you are multiplying both sides by the same number.' Is this the
    necessary and sufficient information needed to solve ohms law for 3
    variables?
     
  7. I use three rules:

    1. If you do something on one side of the equal sign, you must do the
    same thing on the other.

    2. Anything divided by itself equals 1.

    3. Anything multiplied by 1 is unchanged, so the "1" can be discarded.

    I've been working in electronics for some 40 years, and have no idea
    what the "P.I.R.E. wheel" is - I just remember E = IR and P=EI, and
    shuffle things around as needed. The same "shuffling" rules apply to
    any simple equation.
     
  8. Lord Garth

    Lord Garth Guest

    Exactly! And like most technical people, I don't give a rats ass about
    being PC with the resistor color code mnemonic.
     
  9. I can add some to your text above.

    You can do anything to an equation as long as you do it to both sides
    equally, the equation is still valid.
    (an exception is dividing by zero, which gives meaningless results)

    The methods you can use to isolate one variable on one side are
    addition, subtraction, multiplication, division, inverting, squaring,
    square root, substitution, etc..

    Somebody who does not know these methods should take some time to learn
    basic algebra, especially equation solving.
     
  10. Here is such a wheel, if you are curious.

    http://www.the12volt.com/ohm/ohmslaw.asp

    Here is a lesson in simple equation solving algebra

    http://www.algebrahelp.com/lessons/equationbasics/index.htm

    An online equation solver, for really lazy people :)

    http://www.algebrahelp.com/calculators/equation/calc.jsp


    It is incredible how much stuff you can find on the web today, you only
    need to put together the right search words.

    More advanced lessons in algebra. Ask dr Math!

    http://mathforum.org/library/drmath/view/57620.html

    http://www.ifigure.com/math/algebra/algebra.htm
     
  11. js5895

    js5895 Guest

    Thanks, I know basic high school algebra, but I just never understood
    how to apply it to real world problems. I keep reading my electrical
    book on that it says "Current is directly proportional to voltage" and
    "Current is inversely proportional to resistance" and then I look at
    the
    P.I.R.E. wheel, trying to remember the whole wheel just by remembering
    those statements and some algebra. I'm looking at it like a puzzle and
    noticing some patterns like, that the power formulas you have to square
    or square root to find an answer, so I can see that proportional and
    inversely proportional part. I'm trying to figure out how they got
    something like this "I = E/R" from that statement, looking at that
    formula, thinking "I" is proportional to "E" and "I" is inversely
    proportional "R", and I'm thinking why did they divide?. I'm racking my
    mind and I know this is a simple basic DC formula compared to other
    electrical formulas like, the AC ones.
     
  12. Another definitions of "ohms" is volts per ampere. So, for any fixed
    resistance, the ratio of volts divided by amperes (volts per ampere)
    equals the value of the resistance. So resistance is the constant of
    proportionality that relates volts to amperes. 100 ohms means that
    the voltage is always 100 times the amperes.
    The basic definition of resistance R=E/I (ohms equals volts per
    ampere) can be rearranged to I=E/R or E=I*R.

    The second basic formula on those wheels is P=E*I. But you can
    substitute I*R for E (from the above rearrangement of R=E/I) to get
    P=I*I*R or substitute E/R for I to get P=E*E/R

    That is all there is on that wheel.
     
  13. Lord Garth

    Lord Garth Guest

    The term "inverse" means 1/whatever just as the term "per" means "for
    every".
    When someone says "percent" they mean "for every 100". One cent is 1 of
    100.
    As some smart man has said, "Words have meanings". Now if he could only
    pronounce "nuclear" properly.
     
  14. Active8

    Active8 Guest

    And it's so cheap to do these days.

    http://www.sosmath.cfom
     
  15. Active8

    Active8 Guest

    Not to mention very patient or willing to accept no answer at all.

    solve x^3 - x^2 -y = 0 for x :)
     
  16. Rich Grise

    Rich Grise Guest

    At this point, it might help to look at the water pipe model. Voltage,
    or "electromotive force" is pressure, current is the flow rate, and
    resistance is how hard you have to push to get the water to go through
    the pipe. A skinny pipe has more resistance than a fat one.

    The model breaks down when the pipe breaks, and all of your water
    falls out on the ground - that's the opposite of what happens with
    a broken wire; short circuit to "ground" would have that effect. ;-)

    Cheers!
    Rich
     
  17. Wayne Farmer

    Wayne Farmer Guest

    wheel.

    Here's a non-algebra method of deriving the 12 equations of the P.I.R.E
    wheel from just 2 "triangle" diagrams. So, if you can remember the two
    triangle diagrams, you can quickly come up with the whole wheel.

    First, there's the P = IE triangle:

    P
    ------
    I | E

    By covering up either P, I, or E with your finger, what remains will remind
    you of the formula for what you covered up:

    P (covered) = I * E
    I (covered) = P / E
    E (covered) = P / I

    For the remaining 9 formulas of the wheel, start with the E = IR triangle:

    E
    -----
    I | R

    By covering up either E, I, or R with your finger, what remains will remind
    you of the formula for what you covered up:

    E (covered) = I * R
    I (covered) = E / R
    R (covered) = E / I

    Now take the same E = IR triangle, and multiply both the top and left side
    by I. You now get:

    E * I
    -----------
    I * I | R

    Because P = E * I (from the first triangle), and I * I = I^2 (that is, I
    squared), you can rewrite this as:

    P
    ---------
    I^2 | R

    By covering up either P, I^2, or R with your finger, what remains will
    remind you of the formula for what you covered up:

    P (covered ) = I^2 * R
    I^2 (covered) = P / R, so I = square root of (P / R)
    R (covered) = P / I^2

    Now go back to the E = IR triangle, but this time multiply both the top and
    left side by E. This time you get:

    E*E
    ---------
    E*I | R

    This is the same as:

    E^2
    ----------
    P | R

    By covering up either E^2, P, or R with your finger, what remains will
    remind you of the formula for what you covered up:

    E^2 (covered) = P * R, so E = square root of (P * R)
    P (covered) = E^2 / R
    R (covered) = E^2 / P

    You've now developed all 12 equations of the P.I.R.E. wheel.

    --- Wayne
     
  18. phaeton

    phaeton Guest

    Lord Garth said:
    Perhaps i'm nuts to even attempt to fiddle with electronics, because my
    math skills are not so great. I'm 10 years out of HS, and at the time
    i programmed my computer to do my homework for me :-(

    Yeah i know that was stupid. I'm paying for it now.

    Re-teaching myself Algebra is obviously a requirement, but will I need
    to teach myself anything like Calculus? Trigonometry?

    I know that "electronics" is vague, and different parts of it have
    different skill requirements, so take in mind that i'm mainly
    interested in the musical instrument amplification/effects end of audio
    equipment. Logic gates and processing signals like that isn't so
    exciting for me (at least not now)- programming in C has kinda burned
    me out of that sort of thing... :p

    I always consider night classes at the local community college but i'm
    always afraid i'd bomb out of any placement tests and have to start
    math courses from the 7th grade or something. (Which if that's the
    case, then the test worked, and it is pointing me in the right
    direction and telling me exactly what I need to do).

    Although, playing around with some algebra equations i've dug up
    online, i'm surprising myself on some of the things i *do* remember....

    Thanks for any suggestions, even if they're wrong :eek:P

    -phaeton,
    Smoking Si since 1994
     
  19. You can find a lot of help on the web.

    I used the search words
    free algebra lesson
    and found a lot of free resources.

    This lesson in basic algebra, for example:
    http://www.mathleague.com/help/algebra/algebra.htm

    You can probably find useful books in your local library too.
     
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